A technique is known which comprises coating the surface of a powder with another substance to improve the properties of the powder or impart a variety of properties thereto. Various means have conventionally been proposed therefor.
Among powders, metal powders and the like are frequently used because they have various applications. With respect to methods for forming a coating film of a metal on the surface of a metal powder or metal oxide powder, JP-A-3-271376, for example, proposes a method in which a coating film of cobalt metal is formed on the surface of a powder of either a metal such as cobalt metal, nickel metal, or iron metal or a metal oxide such as a ferrite or chromium oxide by reducing a water-soluble cobalt salt by a wet process. However, it has conventionally been impossible to industrially coat the surface of a powder of a metal or metal oxide with a thick and even film of a different kind of metal oxide.
The present inventors previously invented a method for forming a film of a metal oxide by dispersing a metal powder or a metal oxide powder into a metal alkoxide solution and hydrolyzing the metal alkoxide, and filed a patent application (JP-A-6-228604).
The present inventors further developed highly functional powders by alternately forming metal films and metal oxide films on the surface of a metal powder or a metal oxide powder, and filed a patent application (JP-A-7-90310). For example, the inventors succeeded in obtaining a magnetic powder of a sufficiently white color by forming a coating film of a metal oxide on the surface of a powder of a magnetic material such as a ferrite or chromium oxide and forming a coating film of cobalt metal or silver metal thereon, and also in obtaining an insulating powder having satisfactory thermal conductivity by forming a metal oxide film on base particles of a metal having satisfactory thermal conductivity, such as silver metal or copper metal. Furthermore, the present inventors made an application for patent concerning a process which comprises similarly forming a multilayered metal oxide film on the surface of metal or metal compound base particles and subjecting the particles coated with the multilayered metal oxide film to a heat treatment to produce a powder having a denser and stabler, multilayered metal oxide film (JP-A-7-80832).
As described above, the present inventors has made efforts with a view to developing a highly functional metal or metal compound powder by forming one or more films of a metal or metal oxide on the surface of a metal powder or metal compound powder (base particles) to impart a property other than those possessed by the metal or metal compound base particles serving as cores.
It is however desired to provide a more highly functional metal or metal compound powder at low cost and to provide a technique which is applicable also to a wide variety of powders made of materials other than metals or metal compounds, for example, organic powders, and is capable of forming two or more metal or metal oxide films on organic base particles, and the like.
On the other hand, many products especially of nonlinear materials, for example, magnetic materials, such as magnetic heads, magnetic recording materials, and the like, electrical parts, such as boundary layer type capacitors and the like, and glass/ceramic materials, such as high-toughness glass materials, deflecting filters, and the like, are produced from powders or sheets or the like by a method comprising consolidating and molding a powder or by a method comprising superposing sheets and consolidating and molding the same.
Such methods for producing those products include a technique in which an impurity is added to a starting powder and a heat treatment for consolidation is conducted to separate out the added impurity at the boundary between the starting powder and a consolidating material (a medium or the like) to impart nonlinear properties to the consolidated material or utilize the separated substance as a medium for mutually consolidating the powder particles. An example of this technique is a varistor boundary capacitor produced by a method in which a powder of a ferroelectric material such as barium titanate is consolidated and inserted between electrodes made of a metallic conductor, during which operation an insulating impurity is separated from the ferroelectric powder to improve insulating properties.
In glass materials and optical materials, e.g., in MgO—Al2O3—SiO2 glasses, there is a technique in which TiO2 is added to and dispersed into a molten glass prior to molding and the melt is caused to undergo crystallization using the TiO2 as nuclei to give a high-toughness glass material. Another technique comprises dispersing silver into an optical glass to bring the silver into a colloidal state and crystallizing the glass using the colloidal silver as nuclei to give a nonlinear optical material.
Examples of the products produced by superposing sheets and consolidating and molding the same include a magnetic head for recording and reproducing which is produced by superposing sheets of a Permalloy alloy and consolidating the sheets with an adhesive. With respect to other magnetic materials, a material obtained by consolidating a polycrystalline ferrite with an insulator is used as a digital head for audio or computer tapes, while a Sendust alloy is used for tapes having a high coercive force.
In the production of magnetic heads and magnetic cores among the aforementioned magnetic materials and the like, a magnetic material having a predetermined particle diameter is consolidated so as to obtain a maximum energy product to improve magnetic properties. However, it is difficult to conduct consolidation without changing the particle diameter, and the magnetic material unavoidably undergoes an increase in particle diameter as a result of crystal growth therein.
Furthermore, in producing the above-described high-toughness glass material, for example, it is almost impossible to evenly disperse the additive added to a molten glass, which has a high viscosity.
As described above, the production of distinctive products having an added high value, for example, products exhibiting nonlinear properties, necessitates a high level of techniques such as a technique for arranging sheets or acicular base particles serving as cores at the same distance so as to orient in a given direction and consolidating the same while maintaining this state, a technique for bonding the arranged particles by means of films present among the particles, and a technique for evenly mixing particles with a high-viscosity fluid to obtain a homogeneous dispersion. Those products are frequently difficult to produce with conventionally known techniques alone.
An object of the present invention is to eliminate such problems and provide techniques with which distinctive products having an added high value can be stably produced at low cost.
Specifically, the object is to provide, for example, a technique in which magnetic particles having such a particle diameter as to give a maximum energy product are evenly consolidated without causing crystal growth, for example, in order to improve the magnetic properties of magnetic heads and a technique in which high-capacity capacitors on a more constant quality level which are similar in constitution to BL capacitors are produced by employing a conductor as core particles (referred to as base particles), forming a ferroelectric-material layer having an even thickness over the particles, and forming a conductor layer thereon.